Sydney under siege

At least one gunman has taken several people hostage at a cafe in Sydney, Australia.

Pictures on Australian television show at least three people with their hands up against a window, and a black flag with Arabic writing.

Hundreds of armed police have sealed off Martin Place in Sydney's central business district.

New South Wales police have asked people to avoid the area.

An Australian Broadcasting Corporation reporter said that gunfire had been heard at the scene, the Lindt chocolate cafe - but this has not been confirmed.

No injuries have been reported from the incident, according to a police spokesperson.

Police have also said that they are dealing with an "incident" at the Sydney Opera House which has been evacuated.

Local media are reporting that a suspicious package was found there on Monday.

Australian Prime Minister Tony Abbott has described the incident as "deeply concerning" in a statement.

"All Australians should be reassured that our law enforcement and security agencies are well trained and equipped and are responding in a thorough and professional manner," he said.

A National Security Committee of Cabinet has been convened for Monday morning, the PM's office said.

The Lindt Cafe is located in a plaza in the heart of the city's financial and shopping district that is usually packed with shoppers at this time of year.

It is home to the state premier's office and the headquarters of two of the nation's largest banks.

The state parliament house is also only a few streets away.